The present invention relates to the art of electric arc welding and more particularly to a protection device for a dual stage power supply used in electric arc welding.
In electric arc welding and plasma cutting (collectively referred to as xe2x80x9celectric arc weldingxe2x80x9d) dual stage power supplies, such as a dual chopper, are often used to divide the power requirements for the welding operation into two parallel stages. In this manner, a chopper type power supply can provide twice as much electric energy to the welding or plasma cutting operation. The advantage of these parallel choppers are low cost and easy feedback control. However, dual choppers and other dual stage power supplies sometimes create a critical imbalance of the two current paths sharing the output current. The typical causes of this unbalance is poor connections, loss of the switching gate drive in one of the stages, or a failed stage. During such extreme imbalance conditions, one stage of a dual chopper shuts down. The other stage attempts to carry the full load current. This imbalance condition causes a rapid overheating and sometimes destruction of the operating stage. There have been attempts to use electronic monitoring on the dual stages to detect excessive currents through the switching devices. These electronic monitoring devices require appropriate noise immunity and inadvertent nuisance activation. Consequently, electronic monitoring dual choppers and other dual stage power sharing power supplies are quite complex and costly. In the past, a thermostat switch has been mounted adjacent the switching devices to monitor the heat of the switching devices for detecting overheating caused by imbalance. However, it has been found that the thermostat can not respond quickly enough under critical imbalance conditions to thermally protect the switching devices of the power supply. In addition, the thermostat is in the hot ambient area of the power supply. Consequently, the thermostats must be set with a trip temperature above the operating ambient range of the power supply. The ambient temperature rating is xe2x88x9220xc2x0 C. to +50xc2x0 C. For instance, the switching devices of a power supply will typically operate at about 50xc2x0-60xc2x0 C. rise above ambient temperatures. The trip temperature for an imbalance should be in the range of about 70-85xc2x0 C. In this manner, the thermostat will not trip at ambient temperatures, but will trip rapidly at imbalance temperatures. This creates an impossible divergence of operating parameters for the thermostat. Consequently, there is a need for a protection device for dual choppers and other dual stage power supplies which share power for electric arc welding and/or plasma cutting that is insensitive to ambient conditions and rapidly responsive to imbalance of the two stages.
The present invention relates to a detection device for a dual chopper or other dual stage power supplies that includes first and second paths for directing current in unison across an electrode and workpiece of an electric arc welder or plasma cutter. This device includes a resistor connected between generally identical voltage points in the first and second paths of the parallel power supply stages. Consequently, current flows through the resistor based upon the difference in voltage of these identical points. A thermostat mounted adjacent the resistor has a known temperature at which the thermostat will trip to the overheat state. By using the resistor for heating the thermostat, the thermostat itself need not measure the relatively lower temperature excursions of the switches or other components of the parallel power supply stages. Thus, by merely adding a resistance that heats up rapidly between two substantially identical voltage points on the parallel power supply paths, the temperature of the resistor increases rapidly with critical imbalances and rapidly trips the thermostat to either provide a warning signal, deactivate the power supply or perform both of these functions.
In practice, the resistor is connected between the input of a split choke having a common output for directing the two currents from the power supply stage to the electrode and workpiece of the welding operation. By using the inputs of the split choke, a very distinct and positive point of voltage on both stages is obtained. Thus, typical operating voltage differences between the two ends of the appropriately selected power resistor will not cause the thermostat to trip in the highest operating ambient temperature, but voltage differences due to critical imbalance conditions causes rapid temperature increase and fast trip of the associated thermostat. The thermostat trips when there is imbalance of the stages before the switches actually increase to greater than their rated operating temperature. It has been found that in a complete imbalance with one stage inactive, the IGBT switches must be deactivated at a case temperature less than 125xc2x0 C. With the imbalance protection device of the present invention, the thermostat trips in a maximum ambient temperature of 50xc2x0 C. before the rated IGBT switches case temperature is reached. Thus, the protection device switches within a short period of time and before the actual switch of the operating stage of the power supply is overheated. The protection of the present invention trips before the IGBTs case temperature rise exceeds 75xc2x0 C., even when the operating ambient temperature is at the maximum 50xc2x0 C. It has been found that the differential voltage at the input of the split choke is no more than about 0.7 volts during normal balance operation. In extreme imbalance of the power supplies, the voltage difference at the input of the split choke is at least about 2.0 volts. Consequently, there is about a 8:1 wattage increase for heating the detect or resistor for imbalance conditions. In the preferred embodiment, the resistor is two parallel small, high temperature silicon coated one watt wire wound resistors fastened with tight contact to opposite sides of the metal thermostat body. This imbalance detector assembly or unit is then potted into a tube in order to secure the thermal contact between the resistors and the thermostat plate. This potting isolates the assembly from the machine cooling air and provides for insulated mounting and connection with the chopper power supply. The resistor and thermostat unit is mounted on a long extension to have the potted unit spaced from the hot area of the power supply. The thermostat known trip temperature is selected so that the thermostat will not trip when the power supply is operated at 50xc2x0 C. ambient temperature plus a 15-20xc2x0 C. allowance for internal location rise over ambient and normal voltage difference heating. Thus, the thermostat adjacent the resistor will not trip during normal operations of the dual chopper. This feature is enhanced by the isolation and spacing element of the assembled resistor and thermostat unit. Further, the increase temperature of the resistor during normal balanced operation will not trip the thermostat. The thermostat reset temperature needs to be above the rated ambient temperature of the machine. Thus, a thermostat in the preferred embodiment of the present invention is provided with a trip temperature in the range of 70xc2x0 C.-85xc2x0 C. Preferably the trip temperature is 80xc2x0 C. The reset of the thermostat is 55xc2x0 C. which is still above the maximum operating ambient temperature of the power supply. This setting of the thermostat minimizes the required temperature rise to trip the thermostat and avoids nuisance tripping at a balanced temperature approaching 70xc2x0 C. The value of the one watt paralleled resistors was selected to be within its maximum short time overload rating at extreme imbalance. This allows maximum heating rate to the trip temperature of the thermostat. However, when operated at lower voltage imbalance there is a minimum preheating of the thermostat. To accomplish this objective, the two parallel resistors are each 1.0 ohm with a one watt rating. All of these characteristics of the trip and the resistance associated with the thermostat for tripping the thermostat are within skill of the art. The basic concept is the use of a resistor for rapidly heating a thermostat with a known trip temperature so that as soon as there is a critical imbalance, the resistor temperature rapidly increases and trips the thermostat. This is a substantial advantage over merely measuring the temperature of each of the switches in the two parallel stages of the power supply with the further complicated associated with such direct switch temperature measurements when the switches are made up of several matched parallel IGBT packages in a common module.
The primary object of the present invention is the provision of a protection device for a dual stage power supply, such as used in electric arc welding and plasma cutting, which protection device will trip to create a signal quickly upon sensing an imbalance, yet be immune to nuisance thermal or current transient shutdowns.
Another object of the present invention is the provision of a protection device, as defined above, which protection device involves the use of a resistor between generally identical points on the paralleled power supply stages so that with any pre-fault thermal equilibrium condition the temperature of the resistor quickly rises upon a critical imbalance between the two stages to the thermostat trip point faster than the switching device rise to its destructive thermal limit.
Yet another object of the present invention is the provision of a protection device, as defined above, which protection device is easy to manufacture, can be used, or retrofilled on existing power supplies, is positive in operation and not expensive.
Still a further object of the invention is the provision of a protection device, as defined above, which protection device has a potted assembly that can be remotely spaced from the hot area of the power supply.